Variable selectivity control system



Marh 9, 1943. L E THQMPSQN` 2,313,182

` VARIABLE SELECTIVITY CONTROL SYSTEM Filed April 30, 1941 im" wzeoL Y AAA l I "'"IHAVAVAV v v v 42 ,ssamm/vez fet-gama? vide an improved variable tuning capacitor 9 is provided Patented Mar. 9, t

UNI-TED STATI-:s PATENT;

VARIABLE SELEUTIVITY CONTROL SYSTEM Leland E. Thompson, Merchantviile, N.

orto Radio Corporation of America, acorporation of Delaware application April so, 1941', serial No. 391,166

1 claim, (ci. usf-44) electric filter characteristic, the degree of selectivity being variable or adjustable by simple variable resistor means without change in the crystal frequency or other circuit constants. f

It is a furthersobiect of this invention to prof selectivity control system comprising a crystal bridge network having an input and an output circuit and simple'variable resistanceimeans for varying the input or output circuit impedance for the crystal, whereby the degree of selectivity of the network may be varied 20 or adjusted between relatively wide limits.

The invention is particularly adapted for use l in connection with communications receiving A vsystems and, in a vis adapted for use in intermediate frequency and 25 present preferred embodiment,

similar signal amplier sys The invention will, however, be further understood from the following description when considered in connection with the accompanying drawing, and its pended claim. In the drawing, Figure 1 is a schematic circuit ing 'an intermediate frequency amplifier circuit embodying the invention, and Figure 2 is a graph showing curves illustrating certain of the selectivity characteristics of the circuit of Figure 1. Referring to Figure 1, a quartz or other suitscope is pointed out in the apthe primary A also providedwith a shunt tuning capacitor 28,Y

former I6 which is shunted by two series-con'- nected capacitors Il and I8, the midpoint I9 being connected to ground, as indicated at 20. This circuit is connected at one terminal 2l with the input electrode 22 of crystal, and, at the other terminal 23, it is connected through a neutralizing capacitor 24, with the output electrode of the crystal.

Signals are applied to the secondary I 5 through 25 lof the transformer I6, which is and movable ferromagnetic tuning cores represented at 2'Iv are provided for variably adjusting the tuning of both the primary and secondary. Signal input leads for intermediate frequency signals` in the present case are indicated at 2,8.

' The input circuiti is tuned within the intermediate frequency response band, preferably at the intermediate frequency, and provides a relatively hig'h impedance input circuit for the cryslThe illter circuit thus provided is made more selective or sharper by across one of the tuned resistance, the more with various frequencies near resonance, and the selectivityA finally` depends uponl the crystal alone. which normally provides a relatively sharp selectivity characteristic.

Therefore, by variably adjusting the shunt resistance I0 for one of the crystal circuits, such as circuits. 'I'he lowerthis the output circuit- 1, the selectivity characteristic diagram of a signal conveying system comprisable crystal 5 is provided with a signal input cir- 4o cuit 6 and a signal output circuit 'I both of which` are tuned and provided with a variable resistance element in shunt therewith.

In the present example, the output circuit I of the network may be varied f wide range. In

' alternative connection 33, whereby a resistor Il is connected across the output of crystal in place comprising an inductance winding s and a shunt with a shunt variable resistor III. The crystal output electrode II is connected with the high potential side I2 of the circuit l, while the low potential terminal I3 of the circuit 1.

In the present example, the input circuit I is also provided with a variable resistance device 38 which may be connected to operate coniointly lwith the variable resistance I0, as indicatedby is preferably grounded. as indicated. The circuit is variably tunable by mea'ns of a movable ferromagnetic core represented at I4 in conjunction with the inductance 8.

The input circuit 6 comprises the secondary IB `of 'an intermediate' frequency coupling trans- 55 the dotted connection Il. Thus, sistance element may be provided both the input and output circuits a variable rein shunt with over a wider range of variation.

Furthermore. when both' resistors` Il and II are adjusted for maximum resistance. the nu.

reducing the resistance constant the impedanceA is connected with the variable re be coupled to miner oi' the -crystal as an improvement providing-selectivity. control,

cuits have a broad frequency response as indicated by the curve 40 in Figure 2.v This is a desirable characteristic in the reception of voice and modulated signals, as the intelligibility is improved when the side band frequencies are attenuatedto a limited degree. This circuit may be sharpened by reducing the resistance at 3 6 and y l0, preferably jointly, as above described, to approach the crystal selectivity curve indicated at 4I. This curve is also the same with the resistor 36 set at minimum value and the resistor 34 only in the output circuit. A

However, with the resistor 36 maximum, the response is broadest as indicated by the curve 42, this being with the resistor 34 in the output circuit. Thus, when the switch 32 is moved to the contact 33, the resistance load 34 alone is provided on the output circuit of the crystal, while the selectivity characteristic may be varied by changing the value of the resistor 36 alone between that indicated by the curve 4l and that indicated by the curve 42. This circuit has the advantage thatthe input circuit 6 and the trans.- former connected therewith may be an ordinary intermediate frequency transformer adapted for the usual shielding. In this case, the tuned circuit 1 may be omitted, the switch 32 being closed to the contact 33, or the circuit otherwise suitably arranged to include the resistor 34 in shunt acrossb the circuit. A

' It will be noted that by providing hig'h impedance tuned circuits for both the input and output connections for the crystal, the sides of the selectivity characteristic curve 40 are more nearly vertical and the resonance' width of the curve is greater than is possible with other known arrangements affording any extensive degree of selectivity variation, without changing the circonveying or amplier systems in a similar manner. 'I'he variable selectivity control circuit may comprise a high impedance signal source or broadly tuned circuit shunt resistance controlled and feeding a neutralized crystal of the bridge type. The crystal maybe coupled at the output side to a simple resistor or into a broadly tuned circuit across which a shunt variable resistance is provided for sharpening the selectivity as the 4 resistance is decreased as in the input circuit. Thus, the crystal may be preceded and followed by broadly'tuned circuits, each resistance con- `trolled.

The shunt resistor arrangement for controlling the selectivity affects the tuned circuits near the resonant frequency only and thus, at frequencies considerably removed from4 resonance, the circuits 5 and l drop to a low impedance, thus improving the filter characteristic.

This impedance arrangement in series with the series resonance of the crystal at both the input and the output circuits is made variable by simple shunt resistor means for said circuits, whereby the selectivity may be varied from relatively broad to substantially that of the crystal without changing the crystal frequency or other circuit components. i

lclaim as my invention:

An intermediate frequency amplifier circuit comprising,in combination, a coupling transformer having a tuned secondary responsive to a predetermined intermediate frequency, a balanced crystal bridge circuit including a crystal resonant at said frequency connected to the secondary, means providing a variable shunt resistance connection across the secondary 0n the input side of the crystal effectively in series able inductanc'e and a shunt capacitor, andA means providing a variable shunt resistance connection across said last-named circuit effectively in series with the series resonance of said crystal and conjointly variable with said first-named resistance means to increase the selectivity of said network as the resistance of said means is reduced in value.

LELAND E. THOMPSON. 

